PUREFIRE® PFW Series
Hot Water Supply Boiler Sizing Guide
PeerlessBoilers.com
⚠ NOTICE
PB Heat, LLC provides the following information as a courtesy to our customers to aid in the sizing of hot water systems. The
application of this data is the responsibility of the customer or engineer specifying the equipment.
Purpose:
This sizing guide is intended to provide a general method of sizing hot water supply boilers in conjunction with hot water
storage.
Step 1: Determine maximum possible demand
Determine the quantity & type of fixtures to be supported by the hot water supply system. Use manufacturer specifications
for typical fixture usage and flow rate to determine the hourly usage for each fixture.
For example, using a shower head that is designed to provide a maximum of 1.5 gpm of water at 60 psi for an estimated
average shower time of 12 minutes will use 18 gallons of hot water. Therefore, using 18 gph per fixture will be a
reasonable estimated usage for each shower head.
Table 1 shows typical hot water fixture demand values for various types of buildings. Note that the source of the values
predates low-flow fixtures and appliances mandated by the U.S. federal energy efficiency standards in 1992.
Table 1: Hot-Water Demand per Fixture for Various Types of Buildings
Apartment
Club
Gymnasium
Hospital
Hotel
Industrial
Plant
Office
Building
Private
Residence
School
YMCA
(Gallons of water per hour per fixture, calculated at a final temperature of 140°F)
Basin, private lavatory
2
2
2
2
2
2
2
2
2
2
2
Basin, public lavatory
4
6
8
6
8
12
6
-
15
8
3
Bathtubc
20
20
30
20
20
-
-
20
-
30
4
Dishwashera
15
50-150
-
50-150
50-200
20-100
-
15
20-100
20-100
5
Foot basin
3
3
12
3
3
12
-
3
3
12
6
Kitchen sink
10
20
-
20
30
20
20
10
20
20
7
Laundry , stationary tub
20
28
-
28
28
-
-
20
-
28
8
Pantry sink
5
10
-
10
10
-
10
5
10
10
9
Shower
30
150
225
75
75
225
30
30
225
225
10
Service sink
20
20
-
20
30
20
20
15
20
20
11
Hydrotherapeutic shower
-
-
-
400
-
-
-
-
-
-
12
Hubbard bath
-
-
-
600
-
-
-
-
-
-
13
Leg bath
-
-
-
100
-
-
-
-
-
-
14
Arm bath
-
-
-
35
-
-
-
-
-
-
15
Sitz bath
-
-
-
30
-
-
-
-
-
-
16
Continuous-flow bath
-
-
-
165
-
-
-
-
-
-
17
Circular wash sink
-
-
-
20
20
30
20
-
30
-
18
Semicircular wash sink
-
-
-
10
10
15
10
-
15
-
19
DEMAND FACTOR
STORAGE CAPACITY
FACTORb
0.30
0.30
0.40
0.25
0.25
0.40
0.30
0.30
0.40
0.40
1.25
0.90
1.00
0.60
0.80
1.00
2.00
0.70
1.00
1.00
1
20
Note: Data Sources predate low-flow fixtures and appliances.
a
Dishwasher requirements should be taken from this table or from manufacturers' data for model
to be used, if known.
b
Ratio of storage tank capacity to probable maximum demand/hr. Storage capacity may be reduced
where unlimited supply of steam is available from central street steam system or large boiler plant
c
Whirlpool baths require specific consideration based on capacity. They are not included in the
bathtub category.
Adapted from 2011 ASHRAE Handbook - HVAC Applications, Chapter 50, Table 10, Reproduced with permission from American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc.
PUREFIRE® PFW Series
Hot Water Supply Boiler Sizing Guide
The following example shows how to determine the maximum possible demand for a 30 unit apartment
building based on the number of fixtures using hot water:
50 Lavatories
x
2 gph
=
100 gph
30 Showers
x
30 gph
=
900 gph
30 Kitchen Sinks
x
10 gph
=
300 gph
30 Dishwashers
x
15 gph
=
450 gph
10 Laundry Tubs
x
20 gph
=
200 gph
Possible maximum demand
=
1,950 gph
Therefore the maximum possible demand is 1,950 gph. However, it is unlikely that all 30 units will be
using all hot water fixtures at once so, in step 2, we will apply a demand factor to this total.
Step 2: Apply Demand Factor
Apply a demand factor, from table 1, row 19, based on the type of building to which the sizing is to be applied. This is the
probable maximum demand.
Following along with our apartment building example, we use a demand factor of 0.30 based for this type
of building.
Probable maximum demand
=
1,950 gph x 0.30
585 gph
=
Next, we will apply a storage capacity factor.
Step 3: Apply Storage Capacity Factor
Apply a storage capacity factor, from table 1, row 20, based on the type of building to which the sizing is to be applied.
This value will be used to size the hot water supply boiler and storage tank. Tradeoff between boiler recovery capacity
and tank size will be addressed in Step 4.
For our Apartment house example, we will apply a storage capacity factor of 1.25 gallons per gph.
Delivery Capacity
=
731 gallons
=
Step 4: Determine Peak Usage Demand Period and apply
Determine the peak usage demand period for the hot water supply system. For systems in which the peak demand lasts
st
for 1 hour or less, such as some dormitories, the 1 hour capacity should be used (table 2a for PFW-200 and table 2b for
PFW-399). Buildings, such as hotels, with typical peak demand periods between 1 and 2 hours should be sized
nd
according to the 2 hour capacity (table 3a for PFW-200 and table 3b for PFW-399). Buildings with peak demands
rd
between 2 and 3 hours should be sized using 3 hour capacity (table 4a and 4b). The building owner should be
consulted regarding the water usage patterns for the occupants.
In our example, assuming a 2 to 3 hour peak demand period, we can use table 4a to determine the
storage tank requirement if we use a PFW-200 boiler. If the inlet water is 40°F and the water is stored at
140°F then we can use a ΔT of 100°F.
From table 4a:

Selection 1:
PFW-200 with one 119 gallon storage tank
rd
This can be used since the 3 hour rating of 742 gallons exceeds the 731 gallons required.
Using table 4b:

Selection 2:
PFW-399 boiler with one 60 gallon storage tank
rd
This is more than adequate for this application since the 3 hour capacity of 1385 exceeds 731
gallons. In fact, this combination would also be acceptable if the peak demand period were
nd
between 1 and 2 hours (table 3b) since the 2 hour rating is 938 gallons.
rd
We can also calculate the 3 hour rating as follows:
(

Selection 3:
)
(
)
(
)
For a PFW-200 with two 80 gallon storage tanks:
( )
2
PUREFIRE® PFW Series
Hot Water Supply Boiler Sizing Guide
Table 2a: PFW-200 - 1st Hour Capacity (gallons)*
ΔT
Recovery
Capacity
(gph)
Tank Storage Capacity (gallons)
60
80
119
2 x 119
Table 2b: PFW-399 - 1st Hour Capacity (gallons)*
ΔT
Recovery
Capacity
(gph)
Tank Storage Capacity (gallons)
60
80
119
2 x 119
40
549
591
605
633
716
40
1119
1161
1175
1203
1286
60
366
408
422
449
533
60
746
788
802
830
913
80
275
317
331
358
441
80
560
602
616
643
726
100
220
262
276
303
386
100
448
490
504
531
614
Table 3a: PFW-200 – 2nd Hour Capacity (gallons)*
ΔT
Recovery
Capacity
(gph)
Tank Storage Capacity (gallons)
60
80
119
2 x 119
Table 3b: PFW-399 – 2nd Hour Capacity (gallons)*
ΔT
Recovery
Capacity
(gph)
Tank Storage Capacity (gallons)
60
80
119
2 x 119
40
549
1140
1154
1182
1265
40
1119
2281
2295
2322
2405
60
366
774
788
816
899
60
746
1535
1549
1576
1659
80
275
591
605
633
716
80
560
1161
1175
1203
1286
100
220
481
495
523
606
100
448
938
952
979
1062
Table 4a: PFW-200 – 3rd Hour Capacity (gallons)*
ΔT
Recovery
Capacity
(gph)
Tank Storage Capacity (gallons)
60
80
119
2 x 119
Table 4b: PFW-399 – 3rd Hour Capacity (gallons)*
ΔT
Recovery
Capacity
(gph)
Tank Storage Capacity (gallons)
60
80
119
2 x 119
40
549
1690
1704
1731
1814
40
1119
3400
3414
3442
3525
60
366
1140
1154
1182
1265
60
746
2281
2295
2322
2405
80
275
866
880
907
990
80
560
1721
1735
1762
1846
100
220
701
715
742
826
100
448
1385
1399
1427
1510
* The capacities shown above are based on a usable tank volume of 70%. Consult the tank manufacture for the actual usable
volume of tanks.
The calculations used for the capacities above follow:
For PFW-200 with 100°F rise and an 80 gallon tank:
For PFW-399 with 100°F rise and a 119 gallon tank:
For PFW-200 with 80°F rise and two 119 gallon tanks:
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